A threshold of 1 is sufficient for simple_dump_cut.webm, 10 is used
just to be sure the next truncated file doesnt cause the same issue

Obvious alternative fixes are to simply accept that the file is broken or to
write some advanced error concealment or to
simply accept that the decoder wont stop at the end of input.

Fixes: Ticket 8069 (artifacts not the differing md5 which was there before 1afd2469)
Fixes: simple_dump_cut.webm
Fixes: regression of 1afd2469

fate-vp5 changes because the last frame is truncated and now handled
differently.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>